Wednesday, 17 December 2014

A three dimensionally preserved Pterosaur egg from the Early Cretaceous of Argentina.

The Loma del Pterodaustro lake deposits of Central Argentina have
produced large numbers of the Pterosaur Pterodaustro guinazui,
which is interpreted as being a filter feeder with a lifestyle similar to
modern Flamingos, with jaws equipped with specialist filter feeding equipment
and living in large colonies in an environment effectively closed to most other
vertebrates. As well as adult specimens, these deposits have produced a number
of young juveniles and a single eggs specimen, crushed, but with a preserved
embryo inside. To date Pterosaur eggs have only been described from one other
site, the Early Cretaceous Yixian Formation of China.

This second egg does not contain an embryo, but by comparison with
the first egg suggests that has the same dimensions, and since the Loma del
Pterodaustro deposits have never produced any fossils other than Pterodaustro guinazui, Grellet-Tinner et al. feel confident in assigning this
specimen to the same species.

Comparison between the new egg specimen and the
previously discovered, flattened specimen. (a) Photo of the new 3-dimensional
egg in the original claystone matrix with its contouroutlined. (b) Pterodaustro guinazui embryo with the
preserved eggshell outlined. (c) The outline of the pointed section of the
first egg brought to scale and properly orientated matches perfectly the 3
dimensional mold and egg. (d) Traced contour of the original eggshell
tofacilitate comparison between the 2 specimens. Grellet-Tinner et al. (2014).

The new specimen comprises a polar region of the egg, plus adjacent
eggshell, measuring 25 mm in width and 30 mm in length. From this Grellet-Tinner et al. calculate that the egg would
originally have had a volume of 14.7 cm3, and a mass of 15.46 g. The
shell of the egg is preserved without apparent alteration, and is made of
calcium carbonate and shows three separate shell layers, with a total thickness
of about 50 mm. Layer 1 (the innermost layer, and the one that would have been
laid down first) is about 30 mm thick, and is comprised of horizontal
tabular calcitic crystals. Layer 2 (the middle layer) is formed of shorter,
narrower columns of tabular crystals, and is perforated by numerous vesicles,
averaging 2 mm in width. Layer 3 (the outermost layer) is only 2 mm thick, and is composed of amorphous material.

The shell of the new egg is considerably thinner than would be
expected in a Bird’s egg of similar size, which would be predicted to be 179 mm thick. No pores could be found in the shell, which implies that
either it had none, that they are all missing from the eggshell fragments
examined in a scanning electron microscope by pure change (which is judged to
be extremely unlikely) or that they were all concentrated at a the missing pole
of the egg (which seems unlikely, but is known to have been the case in
Troodontid Dinosaurs, and therefore cannot be ruled out).

Pores are important in a mineralized eggshell, as they help oxygen
to reach the embryo and carbon dioxide to escape, while at the same time
regulating the loss of water from the shell. Grellet-Tinner et al. calculate from the size of the egg that if the embryo had
developed at the same speed as that of a Bird, then it would have hatched 21.3
days after it was laid, and during this time could afford to have lost no more
than 14% of the water stored inside to evaporation. Due to the thin shell of
the egg it would have been expected to lose water at a rate of 12.8 mg/d.Torr
(expand), which means that had it been incubated at a temperature of
temperature of 30°C (typical for Crocodilians and Turtles), then the nest would have
needed a minimum vapour pressure of 23.9 Torr in order to retain sufficient
water to survive, while had it been incubated at a temperature of 36°C (typical for modern Birds), then the nest would have needed a minimum
vapour pressure of 36.5Torr (these are minimum values, and it is likely the
humidity in the nest would have been higher than this, to avoid fatal moisture
fluctuations).

Most modern Birds lay their eggs in dry nests, however Grebes
(diving waterfowl closely related to Flamingos) incubate their eggs in moist nests,
and have similar estimated gas conductance’s to the Loma del Pterodaustro egg
(though they do have pores), and the preserved nest of an early Flamingo from
the Miocene of Bardenas in Spain is also thought to have been moist. Mirandornithes
(Grebes and Flamingos) also have an outer layer of amorphous calcium carbonate
around their eggs, something very similar to the outer layer of Loma del
Pterodaustro egg, but not seen in any other Bird. This suggests that the
analogy between the lifestyle of the Pterosaur Pterodaustro guinazui and Flamingos may also be good for its
reproductive behaviour.

Interestingly the preserved Pterosaur eggs of the Yixian Formation
of China do not appear to have been mineralized at all, but leathery like the
eggs of many modern reptiles. This is particularly puzzling as the Yixian
Pterosaurs are thought to have been fairly closely related to Pterodaustro guinazui. While this is
problematic if Birds are used as modern analogies for the lifestyles of
Pterosaurs (all Birds produce mineralized eggs), modern Geckos can produce
mineralized and non-mineralized eggs, often in quite closely related species
living in different environments. Furthermore the mineralized eggs of modern
Geckos (and of fossil Gecko eggs from the Cretaceous of from France) typically
produce shells about 30 mm thick, also comparable to the Loma
del Pterodaustro egg.

Birds evolved from Theropod Dinosaur ancestors in the Jurassic, and
have a fairly extensive Mesozoic fossil record, with around 120 species
described from around the world. The fossil record of Bird’s eggs is...

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About Me

Studied Palaeobiology & Evolution at the University of Portsmouth, Geosciences via the Open University & Ecology and Conservation at Christchurch University, Canterbury.
Have worked in wildlife based tourism, mineral exploration, development, conservation, education & environmental chemistry. Occasionally write articles for papers and magazines.

This Blog would be impossible without the work of countless scientists (and others) throughout the world. Where possible I do my best to credit them, but there will always be many more who remain unmentioned; this does not imply I am ungrateful for their contributions. Any errors or inaccuracies are, of course, my own.